A crystal unit is generally known as a frequency control element which is incorporated in various types of electronic appliance. One type thereof is an AT-cut crystal piece (crystal resonator) in which the resonance mode is the thickness-shear resonance mode, where the resonant frequency band is 2 to 40 MHz. Among such crystal units, beveling is generally used as the crystal piece becomes more compact, to ensure that the resonant frequency is within the 8-MHz band or below.